1. Field of the Invention
The present invention relates to an electron beam system equipped with a spherical aberration corrector.
2. Description of Related Art
Some modern transmission electron microscopes are equipped with a spherical aberration corrector (Cs corrector). The Cs corrector is used to correct spherical aberration (Cs) in an objective lens or the like. One known type of this Cs corrector comprises two multipole elements and transfer lenses disposed between the multipole elements (axisymmetric lens) (see, for example, Japanese Patent Laid-Open No. 2003-92078. Each of the multipole elements produces a hexapole field.
Spherical aberration (Cs) in an objective lens is corrected by such a Cs corrector. However, placing the Cs corrector produces new aberrations (known as parasitic aberrations). Produced parasitic aberrations of the third or less orders include first-order aberration (A1) with two-fold symmetry, second-order aberration (B2) with one-fold symmetry, second-order aberration (A2) with three-fold symmetry, third-order aberration (S3) with two-fold symmetry, and third-order aberration (A3) with four-fold symmetry. It is required that all of these five parasitic aberrations be corrected.
A method of correcting A1, B2, A2, and A3 of these five aberrations is already known. That is, the aberrations A1, A2, and A3 can be corrected if an electrostatic pole element or magnetic quadrupole element, a hexapole element, and an octopole element are respectively used. The aberration B2 can be corrected by utilizing a variation in the electronic orbit caused by a deflection coil. Correction of B2 is also known as coma-free alignment, which was already known prior to achievement of a Cs corrector.
Correction of the aberration S3 has been required by achievement of correction of Cs. However, any method of making this correction has not been concretely established yet, for the following reason. The aberration S3 originates from the fringing term of the quadrupole field. That is, the aberration S3 depends on the term of the quadrupole field differentiated twice by a parameter Z that is the amount of motion of electrons in the direction of motion. Consequently, it is impossible to directly control the aberration using electrostatic or magnetic n-pole elements. The aberration S3 can be corrected by generating and annihilating the fringing term of the quadrupole field at will. Another available method uses an off-axis aberration as S3-correcting means, the off-axis aberration being produced by causing the trajectory of electrons to pass off the optical axis of the optical system (U.S. Pat. No. 6,646,267). However, many other low-order aberrations other than S3 are produced outside the axis. Consequently, the concomitantly produced aberrations other than the intended aberration used mainly to correct S3 produce great adverse effects. Hence, it is not easy to control the system.